you need to get your brake fluid flushed out real soon. that yuck is a mixture of brake fluid, water, rust and dirt. brake fluid attracts moisture, and that will rust out your brake lines and damage the calipers and master cylinder. this is very important. good job on replacing the booster. shows you can do it if your patient and careful.

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1984 300TD "MAX" 303K+ still going...
fast '70 Chevelle 200k+
home built Shovelhead chopper